This invention relates generally to semiconductor lasers, and more particularly, to laser structures in which multiple lasing elements are disposed in a single device to increase brightness and power output. There is currently an urgent need for semiconductor lasers having a peak power capability of one watt, a continuous-wave (CW) output of 100 milliwatts (mW), and a high brightness level. Previously demonstrated semiconductor lasers can emit CW powers of only about 40 mW from one laser facet with high brightness.
By way of background, the basic structure of a p-n junction laser includes an active layer of semiconductor material sandwiched between a p type material and an n type material. A pair of parallel faces of the structure perpendicular to the plane of the active layer, are cleaved or polished, and the remaining faces are roughened to eliminate lasing in directions other than the desired one. The entire structure is called a Fabry-Perot cavity. When a forward bias is applied across the junction, a current flows. Initially, at low currents, there is spontaneous emission of light in all directions. As the bias is increased, eventually a threshold current is reached at which stimulated emission occurs and a monochromatic and highly directional beam of light is emitted from the junction.
Although many different semiconductor laser geometries have been constructed or proposed, lasers of the double heterostructure type are probably the most widely used. In a double heterostructure (DH) laser, the active layer is sandwiched between two inactive layers that take the form of crystalline solid solutions, such as aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As, where x is the fraction of aluminum arsenide in the material. The DH laser has the advantage of being less temperature dependent and operating at a lower current density than a homostructure laser. Also the DH laser provides a greater difference in refractive index at the boundaries between the active and inactive layers, and therefore confines the light more effectively within the active layer.
Various efforts have been made to achieve high power and high brightness from semiconductor lasers, but have met with only limited success. A basic problem in this regard is that merely increasing the emission area of the lasing cavity is not necessarily effective to increase the brightness, because a relatively large cavity tends to operate in multiple spatial modes. In other words, the light source will include multiple lasing spots or filaments. Although more power may be consumed than in a smaller cavity, the light source is spread out among the multiple filaments, and the divergence angle of the resultant beam is increased. Hence the brightness, which is the power per unit source area per unit solid angle of the beam, may not be increased at all. Because merely increasing the size of the laser provides no solution, efforts have been directed to the design of devices with arrays of laser cavities having single-filament lasing properties.
One proposed solution takes the form of a phase-locked array of lasers, as described in U.S. Pat. No. 4,255,717 to Donald R. Scifres et al., entitled "Monolithic Multi-Emitting Laser Device." In this device, multiple lasing cavities are aligned closely together thereby achieving improvement in beam coherence and divergence properties. However, the brightness of such an array is not higher than that of a single laser of the same power, and the far-field light distribution pattern exhibits a twin-lobe shape that is characteristic of a 180-degree difference in phase between individual emitters in the array. The same is true of other multiple laser arrays, such as the one described by D. E. Ackley et al., "High Power Leaky-Mode Multiple-Stripe Laser," Appl. Phys. Lett., Vol. 39, p. 27 (1981).
Most semiconductor lasers employ a relatively narrow stripe of conductive material to make contact with one of the inactive layers, thereby confining the lasing action to the region aligned with the stripe. This structure is also employed in most laser arrays, multiple stripes being employed to define the multiple lasing regions. One exception to this is a CSP large optical cavity laser array device described by D. Botez, in the proceeding of the International Conference on Integrated Optics and Optical Fiber Communication, Paper 29B52, Tokyo, Japan (1983). The structure of this device includes a relatively wide stripe, in the form of a shallow zinc diffusion region, extending over all of the lasing cavities. However, the device is configured to provide high losses in the regions between laser cavites, so that the effect is still similar to that of an array of separate laser diodes.
Other types of laser arrays include the channeled-substrate-planar (CSP) laser, as described by K. Aiki et al., "Transverse Mode Stabilized Al.sub.x Ga.sub.1-x As Laser with Channeled Substrate Planar Structure," Appl. Phys. Lett., Vol. 30, p. 649 (1977). This and other similar array structures provide virtually no coupling between the array emitters, and therefore produce an array of lasing spots, each of which is up to 6 microns (micrometers) in width.
Another difficulty associated with semiconductor lasers is that it is sometimes difficult to maintain stable operation in a single desired longitudinal mode or wavelength. This is particularly true when a laser is modulated with a communications signal. Accordingly an ideal laser for communications applications is one that will maintain a desired longitudinal mode under all conditions.
It will be appreciated from the foregoing that there is still a great need for improvement in the field of semiconductor lasers. In particular, there is a need for a high-power laser that provides a high brightness level and a more desirable far-field pattern. Ideally, such a laser should also provide for stable operation in a single selected longitudinal mode. The present invention provides a novel solution to these needs.